有效核电荷Effective nuclear charge

斯莱特规则给出近似的屏蔽常数：同层电子各屏蔽约 0.35，内一层电子各约 0.85，更内层各 1.00（s、p 电子）。例如钠的 3s 电子：S = 8 × 0.85 + 2 × 1.00 = 8.8，Zeff ≈ 11 − 8.8 = 2.2。粗略地说，外层电子的有效核电荷约等于核电荷减去内层电子数。

Slater's rules give approximate shielding: each electron in the same shell shields about 0.35, each in the next shell in about 0.85, and each deeper one 1.00 (for s and p electrons). For sodium's 3s electron, S = 8 × 0.85 + 2 × 1.00 = 8.8, so Zeff ≈ 11 − 8.8 = 2.2. Roughly, the outer electrons feel the nuclear charge minus the number of inner electrons.

同周期从左到右，核电荷每增加 1，同层电子只增加少量屏蔽，外层电子的有效核电荷增大，所以原子半径减小、电离能增大。同族从上到下，有效核电荷变化不大，但电子层增多、离核更远，所以吸引减弱。

Across a period, each unit increase in nuclear charge is only slightly offset by extra same-shell shielding, so the outer electrons feel a larger effective charge; atoms shrink and ionisation energies rise. Down a group the effective charge changes little but the outer electrons are in higher shells further away, so the attraction weakens.